Probiotics are a group of beneficial microorganisms found in the gut. The World Health Organization (WHO) said probiotics, when taken in adequate quantities, confer a health benefit to the host. The most common probiotic bacteria belong to two genera—Lactobacillus and Bifidobacterium. Other genera included in this group are Enterococcus and Streptococcus, to name a few.
The protective role of probiotics for digestion is now well established. A 2012 meta-analysis of 74 studies, 84 trials, and 10,351 patients showed probiotics are beneficial adjuncts in treating and prevention of gastrointestinal (GI) diseases.(PLOS ONE 2012; 7(11):E48221)
Dietary supplements, pharmaceutical products, medical foods, infant formula, fermented foods such as fresh yogurt, fermented milk, aged cheese, kimchi, craft beer, miso, fermented vegetables like sauerkraut or pickles, sourdough bread, etc., contain probiotic bacteria.
To know more about the fundamentals of nutrition you can read here. A detailed Bibliography is appended at the end that you can readily use for further research.